Solid-state lasers have found applications in all areas where high peak power or high continuous power are required. Examples include material processing (cutting, drilling, welding, marking, heat treating, etc), semiconductor fabrication (wafer cutting, IC trimming), graphic arts (printing, copying), test equipment (confocal microscope), biotechnology instrumentations (proteomics, DNA sequencing, tomography, flow cytometry), medical applications (diagnosis, therapy, micro-surgery), military applications (range finding, target designation), entertainment (laser TV, DVD), and scientific research, to mention a few.
Unlike semiconductor or diode lasers, which are almost always pumped electrically, solid-state lasers based on active ions doped in crystals or glasses are optically pumped. One of the key components of a solid-state laser therefore is an efficient and low-cost light source to provide the optical pumping.
Such optical pumping of solid-state lasers requires the efficient conversion of electrical energy into optical radiation, and an efficient coupling between the generated high-radiation fluxes and the solid-state laser active (gain) medium. Efficient coupling requires a close match between the output spectrum of the pumping source and the characteristic absorption bands of the particular gain medium employed. To maximize the laser output and minimize thermal effects, precise spatial overlap and uniform absorption of pumped photons over the laser mode volume is important.
Flash lamps, arc lamps, laser diodes, and some nonelectric light sources have been employed to pump solid-state lasers over the past years. All of these pumping sources have serious limitations and drawbacks, however.
Historically, flash lamps have been widely utilized for pumping solid-state lasers partly because of their high conversion efficiency. However, due to their non-monochromatic output, the coupling efficiency is generally low. Increasing the flash-lamp's filling pressure could improve the conversion efficiency, however, this would require higher trigger voltage and the simmer current would be more difficult to establish and maintain. The flash-lamp's low coupling efficiency causes a large amount of heat to be generated during the pumping, which limits repetition rates of solid-state lasers pumped by flash lamps. Additionally, the excessive heating leads to undesirable thermal effects such as thermal birefringence, thermal lensing, and even thermal damage. Finally, flash lamps typically exhibit short operating lifetimes, causing frequent replacement necessary.
In sharp contrast to flash lamps, semiconductor diode lasers produce characteristically narrow emissions, which may be advantageously matched to the absorption peak of a laser active medium, resulting in a high coupling efficiency. Unfortunately however, semiconductor lasers are effectively low-peak-power devices and as such, are not best applicable to high-peak-power pulsed mode operations and can be easily damaged by electrostatic discharge or current spikes. In addition, diode laser pumping often does not operate over desirable temperature ranges unless inefficient and oftentimes cumbersome temperature control is used. Moreover, diode lasers have a relatively short lifetime of only 5,000 to 10,000 hours and their cost is high.
Side-pumped lasers typically use cylindrical rods and thus do not exhibit efficient mode-pump overlap, which is particularly problematic for high power scaling. In addition, low dopant percentage has to be used to avoid absorption of pumping energy concentrated near the surface of the laser medium, which may lead to poor overlap between the laser mode and the pumped volume, as well as degradation of the quality of the laser beam due to hot spots inside the gain medium.
Still other attempts were made to pump solid-state lasers with other semiconductor devices, in particular, incoherent monochromatic light sources such as the high-intensity Amplified Spontaneous Emissions (ASE) from rare-earth-doped fluoride, telluride and silica fibers, ASE from super-luminescent diodes, spontaneous emission from Light Emitting Diodes (LEDs), and incoherent or partially coherent emission from Vertical Cavity Surface Emitting Laser (VCSEL) arrays. Among them, LEDs and VCSELs are of particular interest, because their spectral bandwidths may suitably match the absorption spectrum of the lasing medium. In addition, high power LEDs and VCSELs offer some particularly important wavelength ranges, where conventional high power edge emitting laser diodes are unavailable.
VCSEL is a semiconductor device that emits light normal to the device plane with a symmetric beam profile. It has high slope efficiency, is relatively inexpensive and easy to produce. By arranging plurality of VCSEL devices into an array, it is possible to generate high optical power density. In order to serve as a pumping source, the VCSEL array must have a sufficiently large surface area. Three properties, namely, effective heat dissipation, efficient utilization of VCSEL output beam, and uniform injection of the pump energy into gain medium, are thus required.
Heat dissipation may be improved by including a heat sink attached to the device side. In order to decrease the divergence of the VCSEL output beams, beam focusing/collimating elements such as external and discrete lens systems or integrated microlenses may be utilized. For example, in U.S. Pat. No. 6,888,871 and U.S. Patent Application Publication No. 2005/0025211, Zhang et al. have invented a VCSEL device integrated with microlens or microlens arrays and attached with a heat sink. The device emits high-power laser beam with a shape matching the core of fiber optic cables and can be used for pumping fiber lasers such as Er:Yb-doped glass laser.
An innovative solid-state laser device pumped by incoherent or partially coherent monochromatic light sources such as LED and VCSEL arrays has been disclosed by Luo, Zhu, Lu and Zhou in U.S. Patent Application No. 2005/0201442. With this invention, the pump light is efficiently and uniformly coupled into a laser gain medium through a diffusing pump chamber. Compared with LED arrays, VCSEL arrays may have higher optical power and emit light of less divergence. This poses a need for inventing different structures to optimize the coupling efficiency.